1. Field of the Invention
This invention relates to a method of growing a group III-V quaternary or pentanary alloy semiconductor such as InGaAlAs, InGaAlP, InGaAlAsSb, and the like, by a molecular beam epitaxy (hereinafter abbreviated as MBE) process. More particularly, this invention relates to a method of growing a lattice-matched quaternary or pentanary layer on a binary substrate such as InP, GaAs, InAs, and GaSb, or on an epitaxial layer grown thereon, thus forming single or multi-heterostructures.
2. Description of the Prior Art
In a heterostructure having a ternary, quaternary, or pentanary alloy semiconductor grown on a binary substrate or on an epitaxial layer grown thereon, having a different composition, lattice constants of both semiconductors are required to be well matched with each other to obtain high quality epitaxial layers. For example, in a heterostructure in which GaAs/Al.sub.x Ga.sub.1-x As is grown on a GaAs substrate, the epitaxial layer of Al.sub.x Ga.sub.1-x As is considered to be lattice-matched to the GaAs substrate for all x values of 0.ltoreq.x.ltoreq.1. When the lattice constant of the GaAs substrate is represented as a.sub.ga and the difference in the lattice constants between GaAs and Al.sub.x Ga.sub.1-x As is represented as .DELTA.a, then the following condition is maintained for all x values of 0.ltoreq.x.ltoreq.1: EQU a/a.sub.ga .ltoreq.0.13%.
Therefore, epitaxial growth of the GaAs/Al.sub.x Ga.sub.1-x As can be obtained having very few crystal defects over the entire range of x values for Al.sub.x Ga.sub.1-x As semiconductors having different energy band gaps. Therefore, the structure is widely used in photodiodes, lasers, high electron mobility transistors, and the like. In addition, the above-method GaAs/Al.sub.x Ga.sub.1-x As heterostructure is widely used as a structure for opto-devices operating on a wavelength between 0.6 and 0.9 .mu.m.
In another example, in an active device working on a wavelength within a range of 1.0 to 1.6 .mu.m, a quaternary alloy semiconductor such as In.sub.1-(y+z) Ga.sub.y Al.sub.z As (abbreviated conventionally as InGaAlAs) is often grown on an InP substrate or on a buffer layer of a ternary alloy semiconductor deposited thereon. In this case, the deviation of the lattice constant .DELTA.a/a.sub.ip has a maximum value of 3.5% depending on the values of y and z, wherein a.sub.ip denotes a lattice constant of the InP substrate and .DELTA.a denotes a difference between the lattice constants of InP and In.sub.1-(y+z) Ga.sub.y Al.sub.z As.
In the prior art, when the epitaxial layer of a quaternary compound InGaAlAs is grown by MBE forming a heterostructure on an InP substrate, a ternary semiconductor InGaAs, which is lattice-matched with InP, is grown on the substrate, and then a quaternary InGaAlAs, which is also lattice-matched with InP but has a different composition depending on an energy band gap, is grown on the ternary semiconductor. Therefore, a plurality of effusion cells must be provided in the MBE system, each adjusted to generate a specified flux intensity of a molecular beam to grow specified ternary and quaternary epitaxial layers. Each flux intensity is different depending on the composition of the epitaxial layer, even if the same material is utilized as one of the constituents of the epitaxial layers forming the heterostructure.
FIG. 1 is a graph of the flux intensity of each effusion cell versus time in growing a InGaAs/InGaAlAs heterostructure on an InP substrate. In this case, two effusion cells Ga.sub.1 and Ga.sub.2 for Ga are provided in addition to In, Al and As effusion cells. In FIG. 1, a dash-dot line A shows the instant when the MBE growth is changed from ternary InGaAs to quaternary InGaAlAs. During the growth of the InGaAs layer on the InP substrate, three sources are active. The first is an In cell having a flux intensity shown by line (a), the second is a Ga.sub.1 cell having a flux intensity shown by line (b), and the third is an As cell capable of supplying a surplus molecular beam (not shown in FIG. 1).
At the instant shown by line A, a shutter of the cell Ga.sub.1 is closed, and shutters of a fourth cell of Ga.sub.2 having a flux intensity (b') and a fifth cell of Al having a flux intensity (c) are opened, so that the quaternary InGaAlAs epilayer begin to grow. In FIG. 1, the horizontal dashed lines indicate that the shutter is closed. In a case where another ternary or quaternary semiconductor having a different composition from those mentioned above is to be grown continuously to form a multi-heterostructure, additional effusion cells exclusively used for individual layers are necessary in the subsequent processes. Therefore, plural cells generating molecular beams of the same material but having different flux intensities are needed for MBE growth of a heterostructure including a quaternary or pentanary epitaxial layer, because each layer has to have a different composition. Therefore, many cells must be provided inside the chamber.
It is a difficult matter to accommodate a sufficient number of molecular beam cells in the limited space of a MBE chamber in such a geometric arrangement that each cell faces the substrate surface and is substantially equal distance from the substrate. Therefore, if it is required that the effusion cells be reduced to a minimum number, epitaxial growth must be interrupted by closing all shutters immediately after the first epilayer growth, and adjustments of each MBE cell to another flux intensity level are then executed.
The growth can also be interrupted by readjusting the molecular flux intensity by changing a furnance temperature of the effusion cell or the size of the shutter opening before the following growth. These processes, however, are troublesome and precise control is difficult. As a result, the epitaxial layers deposited tend to have crystal defects due to lattice mismatch and to have energy band gaps which deviate from expected values.